As integrated circuits (ICs) increase in complexity, the sizes of underlying IC structures decrease, and the number of layers in the IC increases. This makes the location of underlying structures in ICs more and more difficult. Although the circuit is initially simulated using software, the actual circuit needs to be tested in a number of cases. Such circuit testing may require access to the underlying structures of the circuit.
For example, circuit testing is used when the circuit is debugged, initially during ramp up in production, and in failure analysis. In these cases, individual circuits are analyzed using analytical tools which may include ion mill, electron beam (e-beam), and secondary electron emission (SEM) devices. Such analysis requires finding test points or device signal lines underneath a planarized IC surface. Locating these structures is problematic. Today's techniques generally use particle beam imaging devices, which display only the topography of the die, and not the underlying layers.
In the prior art, locating such test points was wholly dependent on the accuracy of location tools. The top of the die displayed is virtually featureless. FIG. 1 illustrates the only features that can be seen via particle beam imaging devices, the surface metal lines 100. Surface metal lines 100 are generally power lines and global signal lines. The size of an entire circuit area 110 to be located is quite small with respect to the metal lines 100. Additionally, the size of test point 120 or underlying signal line that needs to be located is even smaller. These surface metal lines 100 do not give enough resolution to help find underlying test points 120 or signal lines. Therefore, in order to locate these underlying structures, such as test point 120, in the prior art, the die is placed on a high accuracy and high resolution stage inside an ion mill. The stage is the staging area inside the ion mill or equivalent apparatus, which moves the die to the proper location. This stage moves the die under a high resolution microscope. The movement of the stage must be extremely accurate and high resolution, in the order of fractions of microns, in order to locate the sought features accurately. The accuracy and resolution required for such tools is beyond the scope of most current tools. Any tools that can manipulate the die with such accuracy are extremely expensive. Additionally, even with such high accuracy tools, locating specific signal lines is highly dependent on operator skill and flawless execution. In many applications, the prior art method success rate was approximately 10-20% in locating metal 1 features in a four layer metal device.
The underlying features have to be located accurately in order to test, correct, or adjust them. For example, in the ion mill a buried metal 1 line may be disconnected from or connected to another metal line. In order to adjust such a metal line, without short circuiting it or otherwise interfering with nearby circuit structures, the buried structure has to be located accurately.
Therefore, it can be seen that a simple and highly accurate location method for locating underlying lines or test points in an integrated circuit is needed.